The field of the disclosure relates generally to communication networks, and more particularly, to digitization techniques in access communication networks.
Emerging video-intensive and bandwidth-consuming services, such as virtual reality (VR), augmented reality (AR), and immersive applications, are driving the growth of wireless data traffic in a significant manner. This rapid growth has made the network segment of mobile fronthaul (MFH) networks a new bottleneck of user experience. Various technologies have been proposed and investigated to increase the spectral efficiency of MFH networks and enhance the quality of services (QoS) for end users, such as analog MFH based on radio-over-fiber (RoF) technology and digital MFH based on common public radio interface (CPRI), etc. These conventional proposals, however, have been unable to keep up with the increasing pace of growth of wireless traffic.
In a new paradigm of 5G new radio (5G-NR), heterogeneous MFH networks are proposed to aggregate wireless services from multiple radio access technologies (multi-RATs), and then deliver the aggregated services in a shared ubiquitous access network, as described further below with respect to FIG. 1.
FIG. 1 is a schematic illustration of a conventional access network architecture 100. Architecture 100 includes a core network 102, a baseband processing unit (BBU) pool 104, and one or more remote radio heads (RRHs) 106 (e.g., RRHs 106(1), and mobile users 106(2) and wireless users 106(3), which connect with a respective RRH 106(1)). Architecture 100 is, in this example, a cloud-radio access network (C-RAN) that includes a plurality of centralized BBUs 108 in BBU pool 104 to enable inter-cell processing. Core network 102 includes one or more service gateways (S-GWs) 110, or mobile management entities (MMEs), in operable communication with BBU pool 104 over a mobile backhaul (MBH) network 112. That is, MBH network 112 constitutes the network segment from S-GW/MME 110 to BBUs 108 or BBU pool 104. In a similar fashion, a mobile fronthaul (MFH) 114 is defined as the network segment from BBUs 108/BBU pool 104 to RRHs 106.
In operation of architecture 100, MBH 112 transmits digital bits 116 of net information, whereas MFH 114 transmits wireless services 118 in either an analog waveform 120 based on RoF technology, or in a digital waveform 122 using a digitization interface, such as CPRI. In the embodiment depicted in FIG. 1, architecture 100 represents a heterogeneous MFH network, for aggregating and delivering a plurality of services 124 from different radio access technologies (RATs), including Wi-Fi, 4G long term evolution (4G-LTE), and 5G-NR, to RRHs 106 by way of a shared fiber link 126. Service aggregation of the same RAT (e.g., Wi-Fi channel boning, LTE carrier aggregation (CA), etc.) is referred to as intra-RAT aggregation, whereas heterogeneous aggregation of services from different RATs is referred to as inter-RAT aggregation. A heterogeneous MFH network offers traffic offloading among different RATs and enhances the seamless coverage and provides a ubiquitous access experience to end users.
Accordingly, the conventional MFH technologies include: (1) analog MFH based on RoF technology, which is described further below with respect to FIGS. 2A-B; and (2) digital MFH based on CPRI, which is described further below with respect to FIGS. 3A-B.
FIG. 2A is a schematic illustration of a conventional analog MFH network 200. MFH network 200 includes at least one BBU 202 in operable communication with an RRH 204 over a transport medium 206 (e.g., an optical fiber). BBU 202 includes a baseband processing layer 208, an intermediate frequency (IF) up-conversion layer 210, a frequency domain multiplexer (FDM) 212, and an electrical-optical (E/O) interface 214. In a similar manner, RRH 204 includes a radio frequency (RF) front end 216, an RF up-conversion layer 218, a bandpass filter (BPF) 220, and an optical-electrical (O/E) interface 222.
In operation of MFH network 200, BBU 202 receives digital bits from MBH networks (not shown in FIG. 2A). The received bits are processed by baseband processing layer 208, which provides an OFDM signal to IF up-conversion layer 210 for synthesis and up-conversion to an intermediate frequency. Different wireless services are then multiplexed by FDM 212 in the frequency domain, and finally transmitted through E/O interface 214 to RRH 204 over an analog fiber link of transport medium 206. At RRH 204, after O/E interface 222, the different services are separated by bandpass filter(s) 220, and then up-converted by RF up-converter 218 to radio frequencies for wireless emission. Since these wireless services are carried on different intermediate frequencies (IFs) during fiber propagation, this operation is also referred to as intermediate frequency over fiber (IFoF).
FIG. 2B is a schematic illustration of a conventional analog MFH link 224 for network 200, FIG. 2A. In an exemplary embodiment, MFH 224 represents a system implementation of an analog MFH link based on RoF/IFoF technology, and includes a plurality of transmitters 226 (e.g., corresponding to a respective BBU 202) configured to transmit a plurality of respective signals 228 over link 206. Signals 228 are aggregated by FDM 212 prior to transmission over fiber 206 by E/O interface 214. The aggregated signals 228 are received by O/E interface 222, which provides signals 228 to respective receivers 230 (e.g., of a respective RRH 204). It can be seen from the embodiment depicted in FIG. 2B that the respective RF devices include mixers 232 and local oscillators 234, for both BBUs 202 and RRHs 204, for IF up-conversion and RF up-conversion, respectively. In this embodiment, transmitters 226 are depicted to illustrate the IF up-conversion.
Due to its high spectral efficiency, simple equalization in the frequency domain, and robustness against inter-symbol interference (ISI), orthogonal frequency-division multiplexing (OFDM) has been adopted by most RATs, including WiMAX, Wi-Fi (802.11), WiGig (802.11ad), 4G-LTE (3GPP), and 5G-NR. However, OFDM signals are vulnerable to nonlinear impairments due to their continuously varying envelope and high peak-to-average power ratio (PAPR). Therefore, it has become increasingly difficult to support high order modulation formats (e.g., >256 QAM) using OFDM over MFH networks. To transmit the higher order formats required by LTE and 5G-NR signals without nonlinear distortions, digital MFH networks based digitization interfaces, such as CPRI, has been proposed and implemented. A digital MFH network is described below with respect to FIGS. 3A-B.
FIG. 3A is a schematic illustration of a conventional digital MFH network 300. Digital MFH network 300 is similar to analog MFH network 200, FIG. 2, in many respects, and includes at least one BBU 302 in operable communication with an RRH 304 over a transport medium 306 (e.g., an optical fiber). Network 300 differs from network 200 though, in that network 300 transmits mobile services using digital waveforms over medium 206, which is implemented by the digitization interface of CPRI. BBU 302 includes a baseband processing layer 308, a Nyquist analog-to-digital converter (ADC) 310, a first time division multiplexer/demultiplexer (TDM) 312, and an electrical-optical (E/O) interface 314. In a similar manner, RRH 304 includes an RF front end 316, an RF up-converter 318, a Nyquist digital-to-analog converter (DAC) 320, a second TDM 322, and an optical-electrical (O/E) interface 324.
FIG. 3B is a schematic illustration of a conventional digital MFH link 326 for network 300, FIG. 3A. In an exemplary embodiment, MFH 326 includes a plurality of transmitters 328 (e.g., corresponding to a respective BBU 302) configured to transmit a plurality of respective bit streams 330 over fiber link 306. Operation of network 300 therefore differs from that of network 200, in that, after baseband processing (e.g., by baseband processing layer 308), the waveforms of baseband signals from processor 308 are digitized into bits 330 by Nyquist ADC 310. The digitized bits 330 are then transported to respective receivers 332 (e.g., of a respective RRH 304) over a digital fiber link (e.g., transport medium 306) based on mature optical intensity modulation-direct detection (IM-DD) technology. In the configuration depicted in FIG. 3B, the waveforms of the in-phase (I) and quadrature (Q) components of each wireless service are sampled and quantized separately, and the bits 330 from I/Q components of the different services are multiplexed in the time by first TDM 312. At the respective RRHs 304, after time division de-multiplexing by second TDM 322, Nyquist DAC 320 recovers the I-Q waveforms from received bits 334, which are then up-converted by RF up-converter 318 to RF frequencies and fed to RF front end 316.
Thus, when compared with analog MFH network 200 based on RoF/IFoF technology, digital MFH network 300 demonstrates an improved resilience against nonlinear impairments, and may be implemented by existing digital fiber links, such as, for example, a passive optical network (PON). However, these conventional digital MFH networks suffer from the fact that CPRI has a significantly low spectral efficiency, and may only accommodate few narrowband RATs, such as UMTS (CPRI v1 and v2), WiMAX (v3), LTE (v4), and GSM (v5). Additionally, because CPRI uses TDMs to aggregate services, time synchronization is an additional challenge to the coexistence of multiple RATs with different clock rates. With the low spectral efficiency and the lack of support to Wi-Fi and 5G-NR, CPRI has proven to be a technically-infeasible and cost-prohibitive digitization interface for 5G heterogeneous MFH networks. Accordingly, it is desirable to develop more universal digitization techniques that enable cost-effective carrier aggregation of multiple RATs (multi-RATs) in the next generation heterogeneous MFH networks.